1. Field of the Invention
The invention relates to a hydrodynamic bearing unit supporting without contact a shaft member (or a spindle) by hydrodynamic pressure generated in a bearing clearance. More particularly, the invention relates to a bearing unit suitable for supporting spindles of spindle motors for information-processing equipment. The xe2x80x9cspindle motors in information-processing equipmentxe2x80x9d used herein includes, for example, a spindle motor for driving an optical disk such as CD-R/RWs and DVD-ROM/RAMs, a magneto-optical disk such as MOs, a magnetic disk such as HDDs, a polygon scanner motor employed in a laser beam printers (LBP) or a copying machine, and the like.
2. Description of the Related Art
Conventionally, roller bearings have been commonly used as bearings for supporting spindles of spindle motors for information-processing equipment typified by magnetic disk drives. In recent years, however, roller bearings have been increasingly replaced by hydrodynamic bearings that have excellent characteristics such as high rotational accuracy, high damping, and low noise. A hydrodynamic bearing known (such as Japanese Patent Laid-Open Publication No. Hei. 12-220633, for example) as this kind is such that a shaft member serving as a spindle is supported in radial and thrust directions without contact by generating dynamic pressure in both of clearances, namely, in a radial bearing clearance of a radial bearing portion and in a thrust bearing clearance in a thrust bearing portion.
FIG. 4 shows a known constitution of such a shaft member for the bearing unit where, as described above, both a radial bearing portion and a thrust bearing portion are constituted by hydrodynamic bearings. The shaft member is composed of a shaft portion 21 and a disk-like flange portion 22 which is pressed in and fixed to a shaft end of the shaft portion 21.
In this constitution, however, it is difficult to always obtain highly accurate squareness between the shaft portion 21 and the flange portion 22. This is because the shaft portion is press-fitted into the flange portion. Therefore, end faces of the flange 22 and faces opposed to the end faces may contact with each other during operation of the bearing unit, giving possible affection to bearing performance. Measurement of squareness between the shaft portion and the flange portion after assembling is generally difficult, because they have been already assembled into the unit. Even if it is possible, such work involves complex work to cause increased assembling costs.
Therefore, an object of the present invention is to provide such a hydrodynamic bearing unit that can assure highly accurate squareness between a shaft portion and a flange portion and can be produced at low costs.
Accuracy of squareness between a shaft portion and a flange portion can be improved by forming them in one piece and machining the formed shaft member in one-piece form, while controlling the squareness for a predetermined accuracy. Turning operation is a common machining method for this kind of one-piece type shaft member; however, working by turning consumes long time, sharply increasing working costs.
Further, a surface of the shaft member must be ground for finishing after it has been turned. For this grinding process, a recess portion 24 must be provided at a corner portion 23 between the shaft portion 21 and the flange portion 22 as an enlarged view in FIG. 5 shows. Turning operation is a general working method for forming a portion such as the recess portion 24; however, it is not preferred because it consumes long time for working.
In view of the points described above, a hydrodynamic bearing unit according to the present invention comprises a shaft member having a shaft portion and a flange portion, a radial bearing portion supporting without contact the shaft portion in a radial direction by hydrodynamic pressure generated in a radial bearing clearance, and a thrust bearing portion supporting without contact the flange portion in a thrust direction by hydrodynamic pressure generated in a thrust bearing clearance. The shaft member is formed in one piece by forging, and a recess portion formed by forging or rolling is provided at a corner portion between the shaft portion and the flange portion.
As described above, highly accurate squareness between the shaft portion and the flange portion can be achieved by forming the shaft member in one piece. Further, by forming the recess portions by forging or rolling, working time for the recess portion can be shortened in comparison with the case where the recess portion is made by turning. This, in combination with the fact that the entire shaft member is formed by forging, greatly reduces working costs for the shaft member. In this case, when a recess portion has a special form with an inclination (for example, xcex1=27.5xc2x0, xcex2=27.5xc2x0, xcex8=35xc2x0, and Ro=0.1, as approximate values) as shown in FIG. 5, working by forging or rolling is difficult. Therefore, the recess portion is preferably a non-inclined form without inclination such as a form constituted by a depression portion arranged in an axial direction or a radial direction.
Further, a constitution that can also be taken is such that the shaft member is formed in one piece by forging, and a first recess portion formed by plastically deforming a part of an end face of the flange portion in an axial direction of the shaft member is provided at the end face thereof near an outside periphery of the shaft portion, and at the same time, a second recess portion formed by plastically deforming a part of an outside periphery of the shaft portion in a radial direction of the shaft member is provided at the outside periphery thereof near the end face of the flange portion. In order to form through plastic working the recess portion 24 having an inclined form shown in FIG. 5, a dedicated working device must be developed. However, when the first recess portion and the second recess portion are formed by plastically deforming in axial and radial directions, a working device for general use can be applied, so that a sharp rise in production costs can be prevented.
In this case, the first recess portion may be formed by forging, and the second recess portion by rolling. By doing so, working time can be greatly reduced from the case where the recess portions are formed by turning.
Thus, in this invention, because the entire part of the shaft member including the recess portions is formed by plastic working (forging and rolling), a turning process is eliminated so that working costs can be reduced. Furthermore, forming the shaft member in one piece by forging (and grinding the entire part of the shaft member thereafter) realizes highly accurate finished dimensions (such as squareness), achieving, simultaneously and at high levels, reduced processing costs and increased accuracy.
Thus, high accuracy and reduced costs of a spindle motor for information-processing equipment can be achieved by rotatably supporting a spindle with a hydrodynamic bearing unit having the constitution described above.
The nature, principle, and utility of the invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings in which like parts are designated by like reference numerals or characters.